Their claim is that of the fentanyl seized at the border, 90% of that is from Americans in American cars. The other 10% would be from other sources.

That does not include:

Fentanyl seized at locations other than the border. Which we would know about.

Fentanyl manufactured in the United States.

Fentanyl in hospitals and other regulated areas.

Fentanyl not seized at all.

So they aren't making a claim about all fentanyl. They are making a claim of a certain segment of fentanyl.